Factor VIII (FVIII) is a protein found in blood plasma that acts as a cofactor in the cascade of reactions leading to blood coagulation. Hemophilia A is caused by a reduction or deficiency of functional FVIII protein and is one of the most common bleeding disorders that affects about 1 in 5000-10000 men. Clinical symptoms in hemophilia are frequent muscle and joint bleeds, and trauma can even lead to life threatening situations. Currently, effective treatments for hemophilia include replacing the missing FVIII protein using intravenous application of recombinant or plasma derived FVIII products. Such preparations are generally administered either in response to a bleeding episode (on-demand therapy) or at frequent, regular intervals to prevent uncontrolled bleeding (prophylaxis). Unfortunately, the appearance of neutralizing anti-FVIII antibodies (FVIII inhibitors) is a major complication during replacement therapy with FVIII products. Approximately 25% of the patients receiving treatment develop this immunity to FVIII protein, thus making further control of bleeding very difficult.
The cause for this immune response to FVIII protein has not been fully elucidated, but the specifics of a patient's immune system can affect their response to therapy. Normally, the immune system develops a tolerance to certain antigens, e.g., “self” antigens. This feature is important because, otherwise, if a self antigen is recognized as a foreign antigen, autoimmune disease results. Hemophilia A patients, in particular, have a genetic defect in their FVIII gene, which causes the immune system to not recognize the administered FVIII protein as a “self” antigen. Thus, when FVIII protein is administered during coagulation factor replacement therapy, the patient's immune system recognizes the FVIII protein as a foreign antigen or an altered self protein and develops anti-FVIII antibodies accordingly.
The FVIII inhibitors, i.e., anti-FVIII antibodies are produced by plasma cells derived from FVIII specific B cells. B cells need the help of activated CD4+ T-cells to proliferate and differentiate into anti-FVIII antibody producing plasma cells. For example, FVIII protein is recognized by B and T lymphocytes in different ways. The induction of anti-FVIII antibodies is T helper cell dependent. B cells recognize whole protein epitopes via their specific B cell receptor. T-cells on the other hand, recognize proteins in the form of processed peptides complexed with an MHC class II molecule presented on the surface of an antigen presenting cell. Each CD4+ T-cell clone recognizes only one specific peptide-MHC complex. For presenting the peptides to the T-cells, MHC class II molecules have an open binding groove that allows peptides of various lengths to fit in and be presented on the surface of a cell. Moreover, the MHC class II protein contains four binding pockets that differ for the various haplotypes (Jones et al., Nature Rev. Immunol. 6:271-282 (2006)). Only specific amino acids fit into these binding pockets, and the minimal size of binding peptides is nine amino acids. Notably, different MHC class II haplotypes can present different peptides. Thus, it is likely that a patient's MHC class II haplotype influences the risk of developing anti-FVIII antibodies. Indeed, several studies have shown that there is a correlation of the human MHC class II haplotype HLA-DRB1*1501 with an increased risk for anti-FVIII antibody development (Pavlova et al., J. Thromb. Haemost. 7:2006-2015 (2009); Oldenburg et al., Thromb. Haemost. 77:238-242 (1997); Hay et al., Thromb. Haemost. 77:234-237 (1997)).
Certain approaches have been explored to address the challenges associated with treating hemophilia by administration of FVIII protein. For example, WO 03/087161 discloses modified FVIII proteins, in which the immune characteristics of the FVIII protein are modified by reducing or removing the number of potential T-cell epitopes present on the protein. A number of regions that include T-cell epitopes along the FVIII protein were identified, including, e.g., FVIII2030-2044. According to the disclosure, removal of such regions could be used to provide functional FVIII protein that did not induce production of anti-FVIII antibodies. WO 09/071886 also discloses specific regions of FVIII protein that were predicted to give rise to HLA-DR2 binding peptides that are involved in a patient's immune response, such as, e.g., FVIII475-495, FVIII542-562, FVIII1785-1805, and FVIII2158-2178. The peptides were identified for possible use in inducing immune tolerance in a patient.
While there have been advances in identifying regions of FVIII protein involved in the immune response, there is still a need to identify other regions of FVIII protein that can be used for developing other therapeutic peptides and methodologies that can, for example, be used to treat patients having hemophilia A.